296 
RICHARD M. HOLMAN 
of subsequent elongation, they bent gradually downward until they 
reached the perpendicular. In this respect their behavior was like 
that of roots which had been allowed to grow for from thirty-six to 
forty-eight hours in air and which after attaining the oblique position 
of the elongating zone and curvature of the tip which I have described 
were transferred to loose sawdust without change in position. Al- 
though the rate of downward curvature varied with different indi- 
viduals the final result was always the same, the attainment of a 
vertical position by the growing region of the root. 
The striking behavior just described does not constitute conclusive 
evidence that the roots have become plagiotropic. In Nemec's experi- 
ments and my own the apparently plagiotropic reactions were obtained 
even in the case of roots which had remained in loose moist sawdust 
throughout the experiments and yet eventually roots inverted in 
moist loose sawdust do reach the normal perpendicular position. 
There are other reasonable explanations for the upward curvature of 
roots which we have described than the assumption of a transitory 
plagiotropism by the root. As Nemec (1904, S. 49-50) himself re- 
ported, roots with a distinct tip curvature, when rotated upon the 
clinostat often exhibit oscillating curvatures suggesting somewhat 
those which Baranetzky (1901) observed in the case of shoots. It 
may be that roots with sharply curved tips when released from one- 
sided stimulus by having the tips directed straight downward have the 
same tendency to oscillate as do the roots upon the clinostat. The 
first of these oscillations, if more intense that the succeeding ones 
might be partially fixed when the root was surrounded by sawdust. 
Later oscillations if considerably less intense than the first might be 
almost completely suppressed owing to the resistance offered by the 
medium. If this were the case no permanent upward curvature would 
result in the case of roots in air, a condition borne out by the experi- 
ments which I have reported above, and the slight tendency of roots 
in earth to execute the upward curvature would find its explanation 
in an almost complete suppression of even the first oscillation. ^ 
Whether or not the preceding explanation is correct, it is possible 
to determine whether the upward curvature is a plagiogeotropic 
reaction by transferring roots after they have been inverted in air and 
^ As I shall point out later, the primary geotropic curvature of roots in soil is 
often suppressed by the resistance of the soil until some time after the reaction of 
roots in air has begun. 
